as kids it's easier to see/hear spirits, there's many different theories as to why this is. as you get older, you eyes change, you learn more stuff, you get busy, and for whatever reason most people loose this gift. well, not loose, you've got to work to strengthen it again. look into chakra work, don't focus on just your third eye though, since a fully open chakra, or one stronger than the rest, tends to let energy pool in this spot, and cause disruptions in your bodies natural energy flow. a fully opened and unbalanced third eye can lead to migraines and an overactive imagination [which trust me can be a pain when trying to convince people of things]
work from your root chakra up, going from the crown down can be a bad thing. if you don't have good footing, your spiritual chakra could overwhelm you. [so i've been told] there's also teas and exercises you can use to strengthen your clairvoyant abilities. easiest one to do, take a deck of playing cards, draw one at a time, and try to sense if it's a red card or black card. once you feel confident with this, move on to the suit, then the whole card. each time focus and visualize it in your mind. if you're holding the card and you feel something inside you shouting 'red' don't second guess it, because that's what you're trying for. now, you won't be right 100% of the time, that's fine, but you're trying to strengthen that part of yourself so you can communicate better with spirits.
i can no longer see spirits, but i can sense and hear them. when working with deviation tools, you normally will feel than answer before it appears.
http://www.wikihow.com/Open-Your-Spiritual-Chakras is a great article to help with chakra's in my opinion.